The present invention relates to a controller for a magnetic bearing in an apparatus using the magnetic bearing as a means for supporting a rotor, and more particularly to a controlled magnetic bearing apparatus suitable for suppressing a vibration amplitude in accordance with a whirling movement of an unbalanced rotor.
FIG. 1 shows a basic configuration of a conventionally typical controlled magnetic bearing apparatus having a feedback control system. For easy understanding, in an illustrated example, a part of a bearing apparatus for radially supporting a rotating shaft 1 has been extracted and is designed to control a vibration amplitude of the rotor 1 in an X-axis direction on an X-Y plane (transverse plane) perpendicular to the rotating shaft 1. Specifically, in FIG. 1, the horizontal axis is taken in an X-axis direction, and the vertical axis in a Y-axis direction, about a center of the rotor 1. Displacement sensors 2a, 2b, and electromagnets 3a, 3b are disposed on the X-axis with interposing the rotor 1 therebetween. An electric current to be supplied to the electromagnets 3a, 3b is controlled based on sensor signals from the displacement sensors 2a, 2b. Electromagnets and displacement sensors are similarly disposed on the Y-axis with interposing the rotor 1 therebetween, and an electric current is controlled in the same manner.
As shown in FIG. 1, the displacement sensors 2a, 2b, which are disposed on the X-axis with interposing the rotor 1 therebetween, and which detect radial displacements of the rotating shaft 1, are connected to a sensor amplifier 4. The displacement sensors 2a, 2b and the sensor amplifier 4 constitute a displacement sensor unit. An output signal from the sensor amplifier 4 is an electric signal (sensor signal) corresponding to a displacement of the rotor 1 in the X-axis direction. The sensor signal is inputted into a first control unit 5 for generating a compensation signal utilized for holding the rotor 1 at a desired levitating position.
The first control unit 5 calculates first control signals based on the sensor signal and outputs the first control signals as control currents. The control signals (control currents) are amplified by power amplifiers 6a, 6b respectively connected to the electromagnets 3a, 3b, and then supplied to coils of the electromagnets 3a, 3b. In each of the electromagnets 3a, 3b, an electromagnetic force is generated by the electric current supplied to each of the coils of the electromagnets 3a, 3b. The rotor 1 is magnetically attracted to the electromagnets 3a, 3b by the electromagnetic forces. Thus, in accordance with a displacement of the rotor 1 in the X-axis direction, the control currents are supplied to a pair of the electromagnets 3a, 3b disposed at an opposite position to each other on the X-axis, and hence the rotor 1 is servo controlled so as to be held in a levitated state at a central position or a target position by the attracting forces of the electromagnets 3a, 3b. 
When applications of magnetic bearings become wider, the following problems may arise because of restrictions on their structure, size, and the like:
For example, when a largely unbalanced rotor is rotated while being radially supported by a magnetic bearing, eccentric rotation of the rotor, i.e., whirling, may occur. In such a case, if the degree of eccentricity of the rotor becomes large, then the whirling range of the rotor cannot be within a touchdown gap of the magnetic bearing. Consequently, the rotor cannot be supported in a non-contact levitated state, and this may damage the device.
Further, In the event that a rotor is not levitated at a magnetic center of a motor stator, an external force synchronized with a rotational movement of the motor acts on the rotor. Particularly, in the case of a machine working upon rotation, e.g., a blower, since a load is increased due to an increasing rotational speed, a motor output needs to be increased, and a greater external force synchronized with the rotational movement of the motor acts on the rotor. Consequently, the rotor whirls considerably, and hence a touchdown may occur depending on the degree of the whirling.
Furthermore, when a radial electromagnetic force synchronized with a rotational movement of a motor is generated, a force acting on a rotor as an external force becomes a great load, regardless of a levitating position of the rotor. In this case, as in the aforementioned case, the rotor whirls considerably, and hence a touchdown may occur depending on the degree of the whirling.
In any of the cases, the application of a bearing that can produce a sufficient control power on an external force would solve the problems. However, a stiffness of a magnetic bearing is smaller than that of a rolling bearing or a sliding bearing. Thus, it is difficult for a magnetic bearing to have a stiffness equivalent to that of a rolling bearing or a sliding bearing. For example, when a magnetic flux density of 1 tesla is generated in a space where areas of 1 square centimeter are opposed to each other, an obtained attracting force is about 40 newtons as Maxwell""s stress equation shows. With a controlled magnetic bearing, since a magnetic flux density is generally about 0.5 tesla, an attracting force of only about 10 newtons is obtained.
Accordingly, it has recently been attempted to adopt a feed forward control in which an external force synchronized with a rotational movement of a rotor is estimated, and an input with the addition of a control signal for canceling out the estimated external force is inputted into a power amplifier to thus suppress whirling of the rotor. Further, there has been known an open balance control in which a sine wave or a triangular wave signal synchronized with a rotational speed of a rotor is added to a known external force, and the sum is inputted into a power amplifier to thus control whirling of the rotor. These types of control require not only sensor signals from the displacement sensors 2a, 2b disposed with interposing the rotor 1 therebetween as shown in FIG. 1, but also sensor signals from a displacement sensor for detecting displacements of the rotor 1 in the axial direction of the rotor 1, and pulse signals synchronized with the rotational movement of the rotor 1.
The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a controlled magnetic bearing apparatus which generates a control signal based on a sensor signal from a displacement sensor for detecting a radial displacement of a rotor to suppress whirling of the rotor due to an external force synchronized with a rotational movement, and can hence support the rotor stably in a levitated state.
A voltage signal proportional to a rotational speed, which is obtained from an existing motor controller, is used either for turning on and off a signal switch before a control signal is inputted into a power amplifier, or for operation of a rotational speed component extraction filter.
According to the present invention, there is provided a controlled magnetic bearing apparatus for radially supporting a rotor, comprising a displacement sensor for detecting a radial displacement of the rotor, a first control unit for calculating a first control signal based on a sensor signal from the displacement sensor and outputting the first control signal, a power amplifier for supplying an electric current based on the first control signal, and an electromagnet for generating a magnetic force based on a signal from the power amplifier, the controlled magnetic bearing apparatus further comprising: a second control unit disposed in parallel with the first control unit for generating a second control signal changed in phase from the sensor signal inputted therein and outputting the second control signal; and a signal synthesizer for adding the second control signal outputted from the second control unit to the first control signal outputted from the first control unit to generate a control signal and outputting the control signal to the power amplifier.
The phase change amount in the second control unit is preferably set at a value suitable for suppressing whirling of the rotor, based on external force/displacement transfer characteristics of a magnetic bearing.
With this arrangement, if the rotor whirls, a signal in response to the whirling emerges in the displacement sensor. Thus, in accordance with this signal, a control force in a direction opposite to an external force acting on the whirling is exerted based on the control characteristics of the magnetic bearing, whereby the whirling can be suppressed. The control force is produced by adjusting the sensor signal to a suitable amount of a phase on the basis of the control characteristics of the magnetic bearing. In particular, a phase change amount based on the external force/displacement transfer characteristics of the magnetic bearing is suitable for suppressing whirling of the rotor. Specifically, a set value of a phase adjustor in the second control unit is determined with reference to transfer characteristics (gain, phase) of the sensor signal from the displacement sensor relative to the input signal of the power amplifier in the conventional servo control. The output signal from the second control unit is added to the output signal from the first control unit. The sum is inputted into the power amplifier unit to control an electric current of the electromagnet, whereby whirling of the rotor can be suppressed.
According to the present invention, the second control unit comprises: a filter for extracting a rotational frequency component from the sensor signal; a phase adjustor for adjusting a phase of an output signal from the filter; a signal generator including a comparator for comparing an output signal from the phase adjustor with a reference electric potential; and a gain adjustor for adjusting an amplitude of an output signal from the signal generator. With this arrangement, amplitude information can be cut off from the sensor signal of the displacement sensor, and only phase information can be obtained. Accordingly, an arbitrary gain can be obtained by changing a desired phase amount. Thus, the control force of the electromagnet suitable for suppressing whirling of the rotor can be obtained. It is preferred to use a rotational speed proportional gain adjustor for giving a gain proportional to a rotational speed as the gain adjustor.
According to the present invention, the second control unit comprises: a variable frequency filter; and means for imparting a phase change amount corresponding to a rotational speed of a motor and suitable for suppressing whirling of the rotor. With this arrangement, a sensor signal corresponding to the rotational speed of the motor is extracted by the variable frequency filter, and a phase change amount suitable for suppressing whirling of the rotor is added to the extracted signal. Accordingly, suitable adjustment of the phase amount for an arbitrary rotational speed can be achieved. Thus, whirling of the rotor can be suppressed at rotational speeds over a wide range.
The means for imparting the phase change amount corresponding to the rotational speed of the motor and suitable for suppressing whirling of the rotor preferably comprises a storage for measuring data on external force/displacement transfer characteristics of the magnetic bearing, and storing the measured data in correspondence with a rotational speed, and a phase adjustor for reading from the storage and adjusting the phase. The phase change amount corresponding to the rotational speed of the motor may be set with use of an arithmetic circuit which approximates the external force/displacement transfer characteristics of the magnetic bearing.
According to the present invention, there is provided a controlled magnetic bearing apparatus further comprising: a signal switch for switching on and off a flow of a signal in the second control unit; and a third control unit for comparing the sensor signal with a reference signal, and turning the signal switch on or off based on results of comparison.
According to the present invention, there is provided a controlled magnetic bearing apparatus further comprising: a signal switch for switching on and off a flow of a signal in the second control unit; and a fourth control unit including a comparator for comparing an actual rotational speed signal with a reference signal, and a signal generator for generating a command signal for turning the signal switch on or off.
According to this arrangement, when the rotor whirls greatly, a whirling movement of the rotor can be suppressed by turning on the signal from the second control unit. When the rotor whirls slightly, an ordinary compensation signal from the first control unit is generated by turning off the signal from the second control unit, and hence the rotor can be kept in a levitated state sufficiently.
A second rotational frequency component extractor is preferably provided at the downstream side of a signal generator including a comparator in the second control unit. According to this arrangement, the output signal from the second control unit can be converted into a low-order sine wave by filtering out a harmonic wave component from a rectangular wave. Thus, troubles such as noises due to harmonic waves can be prevented.
The controlled magnetic bearing apparatus preferably comprises a fifth control unit for comparing the sensor output before the signal switch in the second control unit is turned on, with the sensor output at the time when the signal switch is turned on, and outputting a command value for changing a set value of a gain in the gain adjustor. With this arrangement, when the signal from the second control unit is turned on, a gain of the gain adjustor can be set at an appropriate value. Thus, a gain suitable for suppressing whirling can be obtained.